A spectral localizer approach to strong topological invariants in the mobility gap regime

TL;DR

This paper demonstrates that spectral localizer methods can establish the stability and continuity of strong topological invariants in disordered systems with mobility gaps, and shows delocalization of interface states between such systems.

Contribution

It introduces a spectral localizer approach to prove the robustness and stability of topological invariants in the mobility gap regime, extending topological classification to disordered systems.

Findings

Strong topological invariants are stable under homotopies preserving mobility gaps.

The probability distribution of invariants varies continuously in parametrized random systems.

Interface states between systems with different invariants are delocalized.

Abstract

Topological phases of gapped one-particle Hamiltonians with (anti)-unitary symmetries are classified by strong topological invariants according to the Altland-Zirnbauer table. Those indices are still well-defined in the regime of strong disorder when the spectral gap is replaced by a mobility gap, however, many questions regarding their robustness and existence of topological boundary states are wide open. We apply the recently developed spectral localizer method to prove results on the stability of strong topological invariants under a notion of continuous homotopy that preserves a mobility gap condition. Using the local computability afforded by the spectral localizer we show that for parametrized random families that satisfy a fractional moments bound the probability distribution of the strong topological invariant changes continuously. In particular, for ergodic families the almost sure index must be constant on any path which preserves the mobility gap. Using similar methods, we also prove a result on the delocalization of interface states between two mobility-gapped systems which have differing strong invariants.